Fractionation of c{11 {11 {11 bicyclic aromatic hydrocarbons by tetrahalophthalic anhydride complex formation

ABSTRACT

Mixtures of C12 bicyclic aromatic hydrocarbons containing dimethylnaphthalenes are difficult to fractionate by conventional techniques such as distillation or crystallization. However, by dissolving a tetrahalophthalic anhydride in such a mixture at an elevated temperature, followed by subsequent cooling, a solid complex of certain hydrocarbons and the anhydride is formed. Separation of the solid complex from the cooled mixture and its subsequent decomposition results in a product that is substantially richer in those dimethylnaphthalenes which are preferentially complexed. Upon further processing, these dimethylnaphthalenes have utility in the production of dyes.

United States Patent Davis [451 June 13, 1972 COMPLEX FORMATION [72] Inventor: Ronald I. Davis, Wilmington, Del.

[73] Assignee: Sun Oil Company, Philadelphia, Pa.

[22] Filed: May 1, 1970 [21] Appl. No.: 33,949

[52] US. Cl. ..260/674 N [51] Int. Cl ...C07c 7/00 [58] Field of Search ..260/674 N [5 6] References Cited UNITED STATES PATENTS 2,652,435 9/1953 Hess et al... .260/674 2,652,436 9/1953 Hess et al ..260/674 2,914,581 1 1/1959 Christensen et a1. ..260/674 3,249,644 5/1966 Hahn ..260/674 Primary Examiner-Delbert E. Gantz Assistant Examiner-C. E. Spresser Attomey-George L. Church, Donald R. Johnson and Wilmer E. McCorquodale, Jr.

[57] ABSTRACT Mixtures of C, bicyclic aromatic hydrocarbons containing dimethylnaphthalenes are diflicult to fractionate by conventional techniques such as distillation or crystallization. However, by dissolving a tetrahalophthalic anhydride in such a mixture at an elevated temperature, followed by subsequent cooling, a solid complex of certain hydrocarbons and the anhydride is formed. Separation of the solid complex from the cooled mixture and its subsequent decomposition results in a product that is substantially richer in those dimethylnaphthalenes which are preferentially complexed. Upon further processing, these dimethylnaphthalenes have utility in the production of dyes.

6 Claims, No Drawings FRACTIONATION F ct BIQYQLQARQMATLC maocxnaoss BY tmAnAmrn'mmc Mmmtmr: conpu-zx FORMATION C ROSS-REFERENCES TO RELATED APPLICATIONS The present application is copending with the following listed applications filed of even date herewith, all applications being of common ownership.

Ser. Nov lnventor(s) Title carbons by Di or Trianhydride Complex Formation BACKGROUND OF THE INVENTION This invention relates generally to a process for fractionating difficult-to-separate C bicyclic aromatic hydrocarbons and in particular isomers of dimethylnaphthalenes. More specifically, it relates to a process for fractionating isomers of dimethylnaphthalene by initially dissolving a tetrahalophthalic anhydride in the liquid hydrocarbons.

Dimethylnaphthalenes are oxidized to naphthalene carboxylic acids which are used in the production of dyes and pigments. A more detailed discussion of the utility of dimethylnaphthalenes appears in Naphthalenecarboxylic Acids by K. A. Scott in Kirk-Othmer, Encyclopedia Of Chemical Technology, 2nd Edition, Vol. 13.

For convenience dimethylnaphthalene or dimethylnaphthalenes herein will be referred to as DMN, with specific DMN isomers being indicated by reference to the location of the methyl groups. For example, 2,6-dimethylnaphthalene will be referred to as 2,6-DMN.

DMN are found in coal tar, lignite tar, crude oil, the drip-oil fraction produced during the pyrolysis of hydrocarbons to make olefins, in heavy petroleum reformate and in petroleum gas oil produced by catalytic cracking. In these hydrocarbon mixtures the DMN are usually present in rather dilute concentration. For example, one analysis shows DMN making up about 4 percent by weight of a gas oil. However, by known processes such as distillation, crystallization and solvent extraction, DMN can be recovered in concentrated form from the previously mentioned mixtures. In oxidizing these DMN to carboxylic acids, it is usually preferable that each isomer be oxidized by itself since generally each isomer requires slightly different reaction conditions for optimum oxidation.

Most, if not all, the isomers of DMN usually are present in these hydrocarbon mixtures. These DMN isomers, as well as certain other C alkylnaphthalenes such as ethylnaphthalene, have boiling points which are extremely close to each other. This closeness in boiling points makes it extremely difficult to distill apart the individual isomers or distill some isomers from other C alkylnaphthalenes. These boiling points are as follows:

'API Project 44, Tables 23-2-(33.5200), 23 2(33.52l0) and 23-2- (33.5211) Since 2,6-DMN and 2,7-DMN have the same boiling points, these isomers cannot be separated by distillation. Other such pairs include 1,7-DMN and 1,6-DMN, as well as 1,3-DMN and 1,5-DMN.

Certain DMN isomers can be fractionated using the differences in their freezing points. These methods are reported in U.S. Pat. No. 3,202,726 issued Aug. 24, 1965 to E. W. Malmberg et al. and U.S. Pat. No. 3,173,960 issued Mar. 16, 1965 to W. M. Robinson.

However, despite differences in freezing points, DMN form eutectics which make it impossible to further fractionate certain isomers by crystallization. For example, 2,6-DMN with a freezing point of 234 F. and 2,3-DMN with a freezing point of 221 F. form a eutectic with a freezing point of about F. Thus other methods besides crystallization and distillation are needed for separating various DMN isomers from each other.

Other techniques have been suggested for purification and/or separation of DMN. U.S. Pat. No. 3,183,279 issued May 11, 1965 to I. W. Mills et a]. uses selective oxidations. U.S. Pat. No. 3,155,739 issued Nov. 3, 1964 to G. Suld uses a HF:BF complex. Still others have facilitated purification and/or separation by isomerizing various DMN isomers into specific isomers (See J. ORG. CHEM., 29, pages 2939-2946, 1964, G. Suld et al.)

Summarizing, the separation of DMN isomers from each other by known methods is difficult if not practically impossible. There is a need for another purification and/or separation method or a method which facilitates existing procedures.

SUMMARY OF THE INVENTION DESCRIPTION Tetrachlorophthalic anhydride, tetrabromophthalic anhydride, tetraiodophthalic anhydride or tetrafluorophthalic anhydride can be employed as the complexing tetraholophthalic anhydride in the process of this invention. Tetrachlorophthalic anhydride is most frequently employed since it is readily available and is cheapest of the tetrahalophthalic anhydrides. The first of these anhydrides, tetrachlorophthalic anhydride, for convenience will be referred to herein as TCPA. The structure representing TCPA isasfollows:

The tetrahalophthalic anhydride is added, either as a solid or a melt, to the liquid C bicyclic aromatic hydrocarbon mixture containing DMN. The C liquid, during this addition, can be at ambient or elevated temperature. However, if the resulting anhydride-hydrocarbon mixture remains at ambient temperature, i.e., 50-l00 F the rate at which the anhydride dissolves is very slow and the amount ultimately dissolved is very small. The small amount of anhydride that goes into solution at ambient temperature can be recovered as solid anhydridehydrocarbon complex by lowering the temperature of the mixture, e.g., to F. However, the preferred method involves using a temperature which is sufiiciently high so that substantially all the added anhydride will rapidly dissolve. While temperaturesv higher than the normal boiling point of the C bicyclic aromatic hydrocarbon mixture can be used to dissolve the anhydride by employing elevated pressures, it is desirable that a lower temperature be used, preferably below the boiling point of the C mixture and more preferably below the boiling point of the DMN.

After the anhydride dissolves, the solution is cooled to -l50 F., either with or without agitation. During this cooling step, a solid comes out of solution. This solid is a crystalline complex of tetrahalophthalic anhydride and DMN. This solid complex is separated from the liquid by conventional methods such as filtration or decantation.

These complexes are 1r complexes, i.e., they are caused by combination between the nelectrons of the two rings involved. The tetrahalophthalic anhydride apparently accepts a share in the 11' electrons of the compound which is complexed with it. Steric factors appear to have a strong efi'ect, since according to the theory of 11' complex formation, the two rings must be close together and parallel in order for the complex to form. These complexes are distinct from the acid-base type as exemplified by complexes of HF'BF and xylenes and also from clathrate complexes of, for example, the urea-paraffin type.

Some DMN isomers complex more readily than others with tetrahalophthalic anhydride. These differences in tendencies to form complexes can be used to facilitate fractionation of DMN isomers. Furthermore, anhydride DMN complexes can be used to make a wide fractionation. Thus, for example, with TCPA all the DMN with one methyl group in the a-position can be fractionated from the DMN with both methyl groups in the fi-position because of the greater tendency of the former to form TCPA complexes.

The feed to this process can include, in addition to at least two DMN isomers, other compounds that do not alter or destroy the structure of the complex. In general, appreciable quantities of undesirable compounds that will react with the anhydride are to be avoided. Compounds such as C to C alkanes, alkenes, cycloalkanes, cycloalkenes or mixtures thereof are relatively inert and have no appreciable effect upon the complex formation. However, hydrocarbons boiling outside the boiling range of C bicyclic aromatic hydrocarbons and which form complexes with these anhydrides should not be present in appreciable quantities in the feed. For example, US. Pat. No. 2,914,581 issued Nov. 24, 1959 to E. R. Christensen et a]. teaches a process for fractionating mixtures containing a and B methylnaphthalenes using TCPA. Both methylnaphthalenes can be excluded from the feed to the present process by using a feed having only C bicyclic aromatic hydrocarbons containing DMN. Preferably, the C bicyclic aromatic hydrocarbons are C alkylnaphthalenes and ideally they are only DMN.

The amount of tetrahalophthalic anhydride employed in the complexing step can vary over a wide range depending upon the fractionation desired. The amount of anhydride used is related to the amount of DMN present. Preferably, the amount of said anhydride dissolved in the hydrocarbon mixture is in the range from 0.01 to 3.0 moles of said anhydride per mole of DMN. A preferred narrower range is from 0.10 to 1.5 moles of said anhydride per mole of DMN. The contacting of the C bicyclic aromatic hydrocarbons with anhydride can be performed in one contacting stage or a plurality of distinct contacting stages.

The temperature above which the complexing anhydride rapidly dissolves in the hydrocarbon mixture being treated can be determined by simply heating a mixture of the two while agitating and observing the temperature level at which rapid dissolution occurs. I have found, for example, that the preferred temperature for dissolving TCPA in a C bicyclic aromatic hydrocarbon mixture containing DMN is above 300 F., a preferred temperature range being between 325-400 F.

While not all the anhydride added to the hydrocarbon mixture need go into solution, it is preferred that substantially all does. Since anhydride that does not go into solution does not complex with DMN, any undissolved anhydride decreases the amount of complexation per mole of added anhydride and this decrease in complexation per mole of added anhydride is undesirable.

After the tetrahalophthalic anhydride has gone into solution, the solution is cooled with or without agitation until the complex crystallizes out. For TCPA dissolved in a C bicyclic aromatic hydrocarbon mixture containing DMN, this crystallization occurs at temperatures below 300 F. The resulting crystalline complex of DMN-TCPA is a yellow solid.

The solid complex is readily separated from the resulting admixture. Filtration, decantation or centrifugation can be used to remove the complex. Separation of the complex from the treated mixture is ordinarily performed at a temperature below about 150 F.; temperatures between about 50 and F. have proven to be particularly effective for TCPA DMN complex separation. Lower temperatures, e.g., 0 F., can also be used.

After separation, it is advisable to wash the solid complex with a light hydrocarbon solvent in order to remove the physically absorbed liquid mixture from the complex, after which the complex is vacuum dried. Pentane is one example of an excellent wash solvent.

Several techniques can be employed to decompose the solid complex. The preferred procedure comprises heating the solid complex to a temperature whereby the complex is decomposed and the DMN are obtained as a distillate. For a TCPA- DMN complex, this temperature is between 250500 F. The recovered tetrahalophthalic anhydride can be recycled to the same or another contacting zone.

A second procedure for decomposing the complex involves heating the complex in the presence of an inert solvent, such as C to C alkanes, alkenes, cycloalkanes, cycloalkenes or mixtures thereof, whereby the complex decomposes and two layers are formed, one consisting of the complexing agent and the other of a solution of the DMN in a solvent. A temperature of about 200-400 F. is satisfactory for decomposing the complex with a solvent. Accordingly, if a low boiling alkane, for example, is used as a solvent, it may be necessary to use superatmospheric pressure in order to maintain the necessary decomposition temperature. The recovered DMN can be separated from the solution by evaporating the solvent.

A third procedure for decomposing the complex is to contact it with water or an aqueous base, such as aqueous sodium hydroxide. With the aqueous base, the anhydride becomes a salt in the resulting water layer and the organic layer is DMN. The salt in the water layer facilitates the separation of the two layers.

The following example illustrates the invention.

EXAMPLE A water white C bicyclic aromatic hydrocarbon mixture having the composition shown in the following table was treated with TCPA in the following manner. One mole of TCPA per 5 moles of DMN was added to the hydrocarbon mixture. The TCPA used was colorless needles having a melting point of about 492 F. The resulting yellow mixture was heated to about 340 F. with slow agitation. After a solution had formed, i.e., all the TCPA had disappeared, the solution was allowed to cool to ambient temperature. The resulting solid yellow crystalline complex was filtered out of the yellow liquid, washed with inert solvent, viz, pentane, and vacuum dried. The complex was broken by heating with aqueous sodium hydroxide. The water layer contained the sodium salt; the organic layer was the complexate. The complexate was then separated from the water layer and subsequently analyzed.

The following table also shows the hydrocarbon composition for the complexate and the noncomplexate. Also shown are the ratios (weight or mole) of each specific DMN in the complexate to the same DMN in the noncomplexate. If this ratio equals 1, no change in concentration occurred. If this ratio is greater than 1, complexation of the specific DMN was preferential and if less than 1, complexation of the specific DMN was not preferential.

TABLE.FO RMATION OF TCPA-DMN COMPLEXES Weight percent Ratio of C12 bicyclic compound In aromatic complexate liydroto noncarbon Com- N oncomcomplexate Compounds mixture plexnto plexate Other aromatics 28. l) 5. 29. 0 1- and Z-othylnnphtliulenes S. U 2. 2 J. l) 0. 25 5.1! 3.9 5.5 0.71 S. 3 G. .l 8. 6 0. 80 13. a! 25. u 13. 8 1. 89 18. 8 33. 8 17. l 1. 93 14.6 16.2! 12.8 1.32 l. 1 .2. 0 1. 0 2. 00 0. ti 2. 0 l). 4. 00 0. 5 1. 0 0. 4 2. 50 1. 0 1. 4 2. 3 0. 61

Approximately 40 to 50 grams of DMN were complexed per 100 grams TC P a The data in the foregoing table shows that certain DMN complex to a greater extent than others. Thus, for example, the ratio of 1.7-DMN in the complexate to the same DMN in the noncomplexate is 1.93 whereas the ratio of 2,7-DMN in the complexate to the same DMN in the noncomplexate is 0.80.

When other tetrahalophthalic anhydrides, i.e., tetrabromophthalic, tetraiodophthalic and tetrafluorophthaliii. are used with a hydrocarbon mixture such as the feed in the aforementioned table, selective complexing of various DMN isomers likewise occurs. Also, C bicyclic aromatic hydrocarbon mixtures containing DMN, other than the aforementioned feed, can be fractionated in an analogous manner using any of these tetrahalophthalic anhydrides as selective complexing agent.

The invention claimed is:

1. A method of fractionating a liquid mixture of C bicyclic aromatic hydrocarbons containing dimethylnaphthalenes comprising:

a. dissolving in said liquid mixture 21 complexing tetrahalophthalic anhydride selected from the following group: tetrachlorophthalic, tetrabromophthalic, tetraiodophthalic and tetrafluorophthalic; at a temperature between the boiling point of said mixture and F. to complex preferentially with at least one of the dimethylnaphthalenes and form a complex containing less than the total amount of dimethylnaphthalenes in said mixture;

b. cooling the resulting hydrocarbon-anhydride mixture until solid complex of dimethylnaphthalenes and tetrahalophthalic anhydride crystallizes;

c. separating the solid complex from the resulting admixture;

d. and decomposing the solid complex to recover the resulting complexate having a different proportion of dimethylnaphthalenes than the starting hydrocarbon mixture.

2. A method according to claim 1 wherein the tetrahalophthalic anhydride is tetrachlorophthalic anhydride.

3. A method according to claim 1 wherein the mixture of C bicyclic aromatic hydrocarbons consists essentially of a mixture of C alkylnaphthalenes containing dimethylnaphthalenes.

4. A method according to claim 1 wherein the mixture of hydrocarbons consists essentially of a mixture of dimethylnaphthalenes.

5. A method according to claim 1 wherein the amount of anhydride dissolved in said hydrocarbon mixture is in the range of 0.01 to 3.0 moles per mole of dimethylnaphthalenes.

6. A method according to claim 1 wherein the mixture of said hydrocarbons consists essentially of C alkylnaphthalenes containing dimethylnaphthalenes, the tetrahalophthalic anhydride is tetrachlorophthalic anhydride, the amount of anhydride dissolved in said alkylnaphthalenes is in the range from 0.1 to 1.5 moles per mole of dimethylnaphthalenes, the dissolving occurs at a temperature in the range from 300400 F. and the solid complex is separated at 0l 25 F. 

2. A method according to claim 1 wherein the tetrahalophthalic anhydride is tetrachlorophthalic anhydride.
 3. A method according to claim 1 wherein the mixture of C12 bicyclic aromatic hydrocarbons consists essentially of a mixture of C12 alkylnaphthalenes containing dimethylnaphthalenes.
 4. A method according to claim 1 wherein the mixture of hydrocarbons consists essentially of a mixture of dimethylnaphthalenes.
 5. A method according to claim 1 wherein the amount of anhydride dissolved in said hydrocarbon mixture is in the range of 0.01 to 3.0 moles per mole of dimethylnaphthalenes.
 6. A method according to claim 1 wherein the mixture of said hydrocarbons consists essentially of C12 alkylnaphthalenes containing dimethylnaphthalenes, the tetrahalophthalic anhydride is tetrachlorophthalic anhydride, the amount of anhydride dissolved in said alkylnaphthalenes is in the range from 0.1 to 1.5 moles per mole of dimethylnaphthalenes, the dissolving occurs at a temperature in the range from 300*-400* F. and the solid complex is separated at 0*-125* F. 